1. Field of the Invention
The present invention relates to a radioactive isotope production target and a method for fabricating a radioactive isotope production target and more specifically to a 99Mo production target and a method for fabricating a 99Mo production target using low enriched uranium.
2. Background of the Invention
The use of radioactive isotopes is widespread, and includes applications in such diverse fields as industrial flow rate processes environmental investigations and medicine. These radioisotopes are produced primarily by bombarding highly enriched uranium (HEU), or .sup.235 U with neutrons to produce the daughters. While the demand for radioisotopes continues to increase, the use of HEU continues to be discouraged, primarily as HEU can be reprocessed for nuclear weaponry development. Since the United States desires to curtail the export of HEU, it is necessary to find a substitute target material.
One of the major isotopes used in medicine is Technetium-99 m, primarily as this isotope has a short-lived half-life of approximately six hours. Technetium-99 m for medical purposes is a decay product of .sup.99 Mo, which is produced in research reactors from the fissioning of .sup.235 U or from neutron capture in .sup.98 Mo to make the heavier .sup.99 Mo. .sup.99 Mo has a half-life of 66 hours.
.sup.99 Mo is produced using a variety of target designs that contain highly enriched uranium (HEU) of approximately 93 percent .sup.235 U. These designs include cladding shaped as plates, rods and cylinders with uranium material inserted therein. Fuel plate designs utilize a sandwich configuration wherein the fissionable material in the form of a wire or a "meat" matrix resides between two plates of nonfissionable material, such as zirconium, aluminum, nickel, or alloys thereof. The advantage of these designs is efficient heat transfer throughout the target. A disadvantage of these plate designs is the need to dissolve the matrix with high volumes of solution to obtain the fission products. Such processes result in product being lost and/or further decaying prior to use. In addition, many plate designs require the use of highly enriched uranium.
Fuel rod designs (U.S. Pat. Nos. 3,799,883 and 3,940,318) eliminate those losses experienced when processing the products of HEU fission from plate configurations. Such HEU target rods comprise a hollow cylindrical can with a thin layer of UO.sub.2 coated to the inside wall. Molybdenum recovery is accomplished by adding an acid solution into the target cylinder to dissolve the irradiated UO.sub.2 from the cylinder wall for later processing. However, these rod configurations are limited in that they can accommodate only relatively thin layers of UO.sub.2 coating, of approximately 0.001 inches. Such thicknesses can result in reasonable yields of .sup.99 Mo if HEU is employed as the fissionable material, but not if low enriched uranium (LEU) is used. Approximately five to six times the uranium must be processed and recovered for the same .sup.99 Mo yield obtained in HEU processes. However, increasing the thickness of LEU coatings in rod configurations does not work, as flaking of the material off the inside of the cylinder occurs at effective thicknesses beginning at approximately 0.002 inches.
A need exists in the art for a .sup.99 Mo target wherein said target exhibits good heat transfer, has low chemical processing requirements, and incorporates simple design configurations. Such a target must use only low enriched uranium as fissionable material.